1. Field of the Invention
This invention relates to power supplies and more particularly to means included therein which allow two such supplies to be connected to an external load so as to share that load on a substantially equal basis and so as to avoid false indications of supply malfunction arising from that connection at no load.
2. Description of the Prior Art
In many applications, the specifications require that there be redundant power supplies for each load. Quite typically in such applications one of the redundant supplies (hereinafter called the "primary") provides almost the total power requirements of the load and the other of the redundant supplies (hereinafter called the "secondary") serves as a back-up which also provides a small amount of power to the load. Upon the failure of the primary supply, the secondary supply provides the total power requirements of the load. Therefore, arrangements of primary and secondary supplies of the type described above function quite well in many applications by providing the desired redundancy. These arrangements do, however, have some deficiencies.
Upon switching from the primary to the secondary supply, a transient disturbance may be introduced into the load. The arrangements described above tend to maximize that transient and, therefore, its effect on the load as the secondary supply must go from essentially no load to full load when the switching occurs. In addition, while the primary and secondary supplies are identical circuits, the components therein do not age on a substantially equal basis. In general, the components of the primary supply are ordinarily subjected to load operating conditions whereas the components of the secondary supply are ordinarily subjected to only minimal load operating conditions. It has been found that, where the primary and secondary supplies are designed so as to share the load on a substantially equal basis, the transients described above are minimized and the components of each supply tend to age in a substantially equal manner. Therefore, it is desirable that circuitry be included in each supply which allows for the supplies to be connected across the load in parallel in a manner so that there be an almost substantially equal sharing of load between the primary and secondary supplies.
Each supply includes a diode in series with its output. The supplies are each connected to the load through their associated series diodes. The diode functions to disconnect the supply from the load should the supply be turned off as the result of a fault condition occurring therein or for some other reason. Each of the supplies also include leads which are used to sense the output voltage of the supply. The sensed voltage is used by each supply in order to regulate its output voltage. The sense leads are usually connected to sense the supply output voltage at a point prior to the series diode. While this connection allows each supply to regulate its output voltage, it does not take into account the voltage drop across the diode when it conducts or the variation of diode characteristics with temperature. It is also desirable, therefore, that the output voltage of each supply when connected so as to share the load be more closely regulated than is possible when the sensing leads are connected as described above.
In addition, when supplies are connected across the load in parallel so as to share that load on a substantially equal basis it has been found that any difference in their volt-ampere characteristics will give rise to an alarm condition when there is no external load. This is not a true alarm but a false alarm which arises from the sharing of the load by the supplies and the sensing for purposes of voltage regulation that each supply does at the load. Therefore, it is desirable that this false alarm be inhibited at no external load in a manner such that true alarms are not so inhibited.